IGCSE Chemistry Common Ions and Writing Formulae
An ionic formula is built by combining ions so that the positive and negative charges cancel exactly. Knowing the common charges makes every formula in the syllabus derivable rather than memorised.
Formula errors propagate. An incorrectly written formula makes the balancing wrong, which makes the stoichiometry wrong, which loses every subsequent mark. The tables below give the ions the 0620 syllabus uses, followed by the rules for combining them.
Positive ions
| Ion | Formula | Notes |
|---|---|---|
| Sodium, potassium, lithium, silver | Na+, K+, Li+, Ag+ | Group I metals always form 1+ ions |
| Magnesium, calcium, barium, zinc | Mg2+, Ca2+, Ba2+, Zn2+ | Group II metals always form 2+ ions |
| Aluminium | Al3+ | Group III, so 3+ |
| Iron(II) and iron(III) | Fe2+ and Fe3+ | A transition metal with variable oxidation state, so the number in the name gives the charge |
| Copper(II) | Cu2+ | The Roman numeral tells you the charge |
| Lead(II) | Pb2+ | Common in electrolysis questions |
| Hydrogen | H+ | Produced by acids in aqueous solution |
| Ammonium | NH4+ | A positive ion that contains no metal |
Negative ions
| Ion | Formula | Notes |
|---|---|---|
| Chloride, bromide, iodide | Cl-, Br-, I- | Group VII elements form 1 minus ions |
| Oxide and sulfide | O2- and S2- | Group VI elements form 2 minus ions |
| Hydroxide | OH- | Produced by alkalis in solution |
| Nitrate | NO3- | All nitrates are soluble |
| Carbonate | CO32- | Reacts with acids to give carbon dioxide |
| Sulfate | SO42- | Tested with acidified barium nitrate |
| Nitride | N3- | Group V, so 3 minus |
Rules for building a formula
| Step | What to do | Example |
|---|---|---|
| 1 | Write the positive ion first with its charge, then the negative ion with its charge | Mg2+ and Cl- |
| 2 | Find how many of each are needed for the charges to cancel | One Mg2+ needs two Cl- |
| 3 | Write the numbers as subscripts, omitting any 1 | MgCl2 |
| 4 | Use brackets when more than one of a compound ion is needed | Ca(OH)2, not CaOH2 |
| 5 | Check the total charge is zero | 2+ and two lots of 1 minus cancel |
| 6 | Balance the equation by changing the large numbers in front, never the subscripts | 2Mg + O2 gives 2MgO |
How to use this sheet
- The group number gives the charge for main group elements. Group I is 1+, Group II is 2+, Group VI is 2 minus and Group VII is 1 minus.
- A Roman numeral in a name is telling you the charge. Iron(III) means Fe with a 3+ charge.
- Brackets are compulsory when more than one compound ion appears. Ca(NO3)2 has two nitrate ions, CaNO32 means nothing.
- Never change a subscript to balance an equation. Doing so changes the substance rather than the quantity.
- Count atoms inside brackets when calculating a relative formula mass. Ca(OH)2 contains two oxygen and two hydrogen atoms.
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Frequently asked questions
How do I write the formula of an ionic compound?
Write both ions with their charges, then work out how many of each are needed for the charges to cancel exactly. Write those numbers as subscripts, omitting any subscript of one, and use brackets whenever more than one compound ion is required.
What does the Roman numeral in a name mean?
It gives the charge on the metal ion, and it is used for transition metals because they have variable oxidation states. Iron(II) is Fe with a 2+ charge and iron(III) is Fe with a 3+ charge, which produce different formulae and different coloured compounds.
When do I need brackets in a formula?
Whenever more than one of a compound ion such as hydroxide, nitrate, carbonate or sulfate is required. Calcium hydroxide is Ca(OH)2 because two whole hydroxide ions are needed. Without brackets the subscript would apply only to the hydrogen.
Why can't I change subscripts to balance an equation?
A subscript defines what the substance is. Changing it produces a different chemical, so the equation no longer describes the reaction asked about. Balancing is done only by adjusting the large numbers written in front of each formula.
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Written to the published Cambridge IGCSE syllabuses. Always check the formula list and data sheet issued with your own paper, since the material provided differs between subjects and syllabus versions. Last reviewed 2026-08-12.